In the production of hydrocarbon fluids from subsea formations, it is often desirable from an economic standpoint to achieve early production of the hydrocarbon fluids prior to the installation of a more permanent, bottom-founded structure. Additionally, formations are often discovered which are marginally economic for the installation of a high cost, permanent production structure. For these reasons, production of the subsea hydrocarbon fluids to a floating production facility is often considered.
Floating hydrocarbon production facilities have found application for development of marginally economic discoveries, early production, extended reservoir testing, and flexibility in offshore development. Additional advantages of a floating facility over a conventional platform include early production and cash flow from one to two years ahead of a fixed platform as well as lower initial cost. Further, upon depletion of the produced field, a floating facility may be easily moved to another field for additional production work.
In deeper water (300 feet or more), the use of bottom-founded steel or concrete structures for oil well drilling and production operations becomes quite expensive due to the high cost of fabrication and installation of such large structures. In deep water, construction and installation times are extended which delays the onset of revenue from production. Moreover, oil reserves in place must be much larger in deep water in order to justify the higher development costs. The number of "marginal" subsea hydrocarbon fields grows rapidly with increasing water depth.
Floating production systems employing ship-shaped vessels, barges or semi-submersible-type hulls have been used to obtain "early production" prior to construction of permanent, bottom-founded structures. Floating production systems have been installed to produce "marginal" subsea reservoirs with only a few wells, reservoirs to small to develop with bottom founded structures.
Existing floating production systems utilize various types of production risers to convey produced fluids from the sea floor manifold or subsea wellhead to the surface. Because the risers are commonly steel pipe and are fixed at their lower end, they must be supported at their upper ends with automatic heave compensating equipment so that vertical vessel motions (heave) produced by wave action or tidal effects are not imposed on the production risers. Additionally, because of the floating vessel cannot be held in an exact surface position by the mooring system, the lower end of a production riser must be equipped with a flexible connection to prevent the development of bending loads in the riser or subsea wellhead as a consequence of vessel excursions away from a surface position directly above the lower connection of the production riser.
Flexible piping such as disclosed in U.S. Pat. Nos. 3,499,668, 3,559,693 and 4,213,485 have been used in various offshore installations for transport of hydrocarbon fluids. Flexible piping has been laid on the sea floor as flowlines to connect individual subsea wells to a centrally located sea floor manifold. Further, flexible piping has also been used to convey produced fluids from the sea floor to the surface. Commonly, these flexible risers have been configured into a catenary pattern. To obtain such a configuration, installations have been made in which the flexible pipe has a single buoy or a lay on the sea floor intermediate the sea floor connection and the surface facility such as a ship, semi-submersible, or buoy. Such configurations do not allow a direct "wireline" reentry into the production strings for well servicing. Typical of such an installation is that described in U.S. Pat. No. 4,266,886.
In the afore-mentioned U.S. Pat. No. 4,266,886, heave compensation is provided for in a flexible production riser without the use of mechanical heave compensators by the provision of a catenary loop in the flexible riser. It is also known to position floatation means at the wellhead side of the catenary loop in order to maintain the catenary flexible riser out of contact with the sea floor. Alternatively, it is known to lay a substantial length of flexible riser directly on the sea floor which is picked up off the sea floor in compensating for vertical heave of the floating facility to which it is connected. None of these apparatus allow for the use of wireline well service tools since such tools cannot pass by gravity through a loop or horizontal lay in the riser.
It is also common to provide various floation means on subsea production risers principally for the purpose of reducing riser weight. U.S. Pat. Nos. 3,605,413, 3,768,842, 3,952,526 and 3,981,357 are exemplary of this type of light weight, metallic riser incorporating floation. Floation has also been used with flexible production risers as illustrated in U.S. Pat. Nos. 3,517,110 and 3,911,688.